This invention relates to a method and system for automatically detecting and correcting marginal data transmissions in a polling loop system used for security system communications.
Security systems that comprise a number of devices interconnected to a control panel by a communications bus, are well known in the art. Security devices typically are used to monitor an area of space or a specific access point, and report to the control panel if there is a change in status. For example, devices exist that monitor opening of doors or windows, that determine if an intruder has entered the premises such as by passive infrared surveillance techniques, or that determine if a fire has started, etc. Since most of these types of devices only report changes in status when a triggering event occurs, and a triggering event such as a fire may never in fact occur, it is important to poll or query each device on some periodic basis in order to ensure that they are up and running. This polling process is referred to as supervision of the devices, and generally is carried out by the control panel querying each device individually to determine at least if it is capable of sending a response back to the panel. If any given device does not report back, then the system will provide a warning to the system operator or monitoring company that the device needs to be investigated.
In order for a polling loop system to operate properly within acceptable margins, it must be installed within specific guide lines as defined to an installer via the associated installation instructions. Guidelines such as wire gauge, wire length, the use of shielded or non-shielded cable or metal conduits, number of devices per loop, etc. must be followed by the installer. However, it is more often than not that the installer badly estimates the length of the various wire he has installed and the allowable number of allowable devices installed which is within the power drive capability of the system. Exceeding the allowable wire size and capacitive and/or current loading in a given installation can result in unreliable signal recovery and can create intermittent or permanent errors which the installer has difficulty correcting.
This invention provides the security control unit with the ability to ascertain the overall transmission and reception margins of the system after the installation either by a special command from the installer or automatically by the security panel effected periodically while the system is in actual use. The polling loop system is used here as an example of an addressable polling loop system.
The invention is based on multiple time sampling of the received signals wherein at least one time sample occurs within the acceptable range of the received signal and at least one time sample which is outside the acceptable range of the received signal. If one or more devices are determined by the security control to be outside acceptable limits, the security control can adjust the baud rate of all, or specific, devices on the loop to bring those devices within marginal limits and/or to notify the installer via suitable keypad ennunciations.
Thus, the present invention is a system and method for detecting marginal data transmissions from any of a number of security devices in a security system including a control unit in communications with the security devices over a serial data communications loop. First, the control unit receives a data transmission from a security device, wherein the data transmission includes a number of bit intervals in which a logic 1 level is assumed by the control unit unless a logic 0 level is detected by the control unit. The control unit samples the data transmission at a first predetermined time during the bit interval to obtain a first sample value, and then it samples the data transmission at a second predetermined time during the bit interval to obtain a second sample value (the second predetermined time being later than the first predetermined time). If the first sample value is a logic 1 and the second sample value is a logic 0, this indicates that the data transmission from the security device is marginally recoverable.
If, however, the first sample value is a logic 0 and the second sample value is a logic 0, this indicates that the data transmission is acceptable. If the second sample value is a logic 1, then the control unit assumes the transmitted data bit to be a logic 1, and it makes no indication regarding the acceptability of the data transmission.
Optionally, prior to sampling the data transmission at a first predetermined time during the bit interval, the control unit may take a pre-sample at a third predetermined time prior to the first predetermined time. If the pre-sample value is a logic 1, the first sample value is a logic 0, and the second sample value is a logic 0, this also indicates that the data transmission is acceptable with marginal distortion.
If the data transmission has been indicated to be marginally recoverable, then the control unit may lower the baud rate of transmission between that security device and the control unit by a pre-determined increment in order to alleviate the marginal data transmission problem.
For example, the third predetermined time may be approximately ⅓ through the duration of the bit interval, the first predetermined time is approximately ⅔ through the duration of the bit interval, and the second predetermined time is near the end of the bit interval.